Power supply systems are pervasive in many electronic applications from computers to automobiles. Generally, voltages within a power supply system are generated by performing a DC-DC, DC-AC, and/or AC-DC conversion by operating a switch loaded with an inductor or transformer. In some power supply systems, combinations of switches are arranged in a bridge configuration such as a half-bridge, full-bridge, or a multi-phase bridge. When very high voltages are generated by the power supply, it is beneficial to use switches, such as junction field effect transistor (JFET) devices, that have both a high breakdown voltage and a low on-resistance. The high breakdown voltage of a JFET allows for reliable operation even with output voltages of hundreds or even over a thousand volts. The low on-resistance of the JFET device allows for efficient operation of the power supply system.
JFET devices have the property that they are self-conducting or “normally on devices,” meaning that the devices conduct electricity when the gate-source voltage of the JFET is at about zero volts. Such a property poses difficulties because the switch transistors appear as short circuits before the power supply system is fully biased, thereby causing high currents to be generated at the startup of the power supply. In some high efficiency JFET devices used for power supply switching, this pinch-off voltage may be around negative 15 volts. Therefore, this negative voltage is generated before the power supply begins full operation when the JFET can be fully shut off.
In some power supplies, biasing voltages are developed at startup by using transformers. The use of transformers, however, is expensive. In other power supplies, voltages are developed at startup by using bootstrap techniques, in which the energy of switching nodes within the power supply circuit is used to charge capacitors that provide the local power supply for the switching transistors. When JFETs are used, however, such bootstrapping techniques are difficult to apply. For example, when the internal supply voltage of the power supply system is low at startup, the JFET switches may not operate because the voltages required to allow the JFETs to switch on and off has not yet been developed. If the JFET switches do not operate, then the internal supply voltages needed to make the switches operate cannot be generated.